This invention relates to a light beam scanning device which scans at a high speed a light beam emitted from a light source such as, for example, laser beam, etc., by deflecting and modulating the same.
In the light beam scanning devices which deflects and scans a light beam from a light source using a deflector such as a rotatory polygonal mirror or a galvano-mirror to thereby scan a scanning surface, it is desired that intensity of a beam spot on the scanning surface have a substantially uniform value at every point on the scanning surface, and that moving speed of the beam spot be substantially constant.
Various arts of moving the beam spot at an equal speed have heretofore been proposed. U.S. Pat. No. 3,573,849, for example, teaches to use an image forming lens for scanning, in which the focal length thereof changes depending on an incident angle of the beam. Further, there have been proposed techniques of varying length of modulating signal input into the modulator, or controlling a rotational angle of the deflector.
On the other hand, in the conventional beam scanning device, for the light beam to be deflected by a rotatory polygonal mirror, and for the thus deflected light beam to be focused on the scanning surface in the form of a spot to scan the surface of a scanning object, use is made of a y=f tan .theta. lens or an f.multidot..theta. lens (where: y denotes a distance from the optical axis of the image forming lens to an image forming position of the light beam; f is a focal length of the image forming lens; and .theta. is an angle formed by the optical axis of the image forming lens and the light beam entering the image forming lens) as an image forming scanning lens. On account of this, when the incident angle of the beam into the image forming lens becomes large, the spot intensity at the peripheral region of the scanning surface becomes remarkably low in comparison with the spot intensity at the center part thereof with the consequence that irregularity in image density takes place inconveniently. Therefore, when a spot having an intensity of a certain definite value and above is to be obtained over the entire scanning surface, it becomes necessary that the low spot intensity at the peripheral region on the scanning surface be taken as a reference, for which purpose a high output light source as disclosed in a laid-open Japanese patent application No. 52-99836 should be used.